Well i think its like this.
Ur char is wearing the lightning damage increasing items not your traps.
So the items will only increase the lightning damage that comes from your assassin (shockweb) and not the lightning damage that comes from your traps. Well i,m sure the most people already know this but i couldend resist to post this.

hmm.. well it seems like im doing more dmg with ormus robe. guess its only a bug then =/. but who said that the +light % dmg dont works with the lightsentry??? :O

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I'll put it simply for you. If you are a summon necro, would stacked DS and CB help your skellies? No, it wouldn't, because that would only affect you.
Traps are Minions, unlike you think, thereafore all that +%lightning skill damage and -enemy resist does zilch for Death Sentry, Lightning Sentry, Charged Bolt Sentry, Wake of Fire and Wake of Inferno. However, it does affect shock web and fireblast.

Just try Overly stacking +%lightning skill damage and -enemy resist (Griffons, 6facet Onehanded weapon, 4facet Monarch, facet Ormus Robes, and the like) and not having too much plus skills (screw your skillers ).
Then go up against someone with 75 lightning resist and high life. Test the damage by casting a single Lightning Sentry (recast when it runs out) and counting the hits. After you get 30 or so hits (maybe 20, I dunno), divide the damage done to the other character by the number of hits, then multiply by 24. The damage will be FAR lower than the average damage expected from the numbers on the LCS (Lying Character Screen).

I'll put it simply for you. If you are a summon necro, would stacked DS and CB help your skellies? No, it wouldn't, because that would only affect you.
Traps are Minions, unlike you think, thereafore all that +%lightning skill damage and -enemy resist does zilch for Death Sentry, Lightning Sentry, Charged Bolt Sentry, Wake of Fire and Wake of Inferno. However, it does affect shock web and fireblast.

Just try Overly stacking +%lightning skill damage and -enemy resist (Griffons, 6facet Onehanded weapon, 4facet Monarch, facet Ormus Robes, and the like) and not having too much plus skills (screw your skillers ).
Then go up against someone with 75 lightning resist and high life. Test the damage by casting a single Lightning Sentry (recast when it runs out) and counting the hits. After you get 30 or so hits (maybe 20, I dunno), divide the damage done to the other character by the number of hits, then multiply by 24. The damage will be FAR lower than the average damage expected from the numbers on the LCS (Lying Character Screen).

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Ok..but now .. i really dont care ... im owning many assas with enig and so on.... and im just using cheap stuffs..